FIG. 8 illustrates an example of conventional structure of a steering apparatus for applying a steering angle to steered wheels (front wheels). In this steering apparatus, a steering shaft 3 is supported on the inner diameter side of a cylindrical shaped steering column 2 that is supported by a vehicle body 1 so as to be able to rotate freely, and a steering wheel 4 is fastened to the rear end section of the steering shaft 3 that protrudes toward the rear further than the opening on the rear end of the steering column 2. When the steering wheel 4 is rotated, the rotation is transmitted to an input shaft 8 of a steering gear unit 7 by way of the steering shaft 3, a universal joint 5a, an intermediate shaft 6 and a universal joint 5b. As the input shaft 8 rotates, a pair of tie rods 9 that are located on both sides of the steering gear unit 7 are pushed or pulled, which applies a steering angle to a pair of steered wheels according to the amount that the steering wheel 4 is operated. In the case of the structure illustrated in FIG. 8, a telescopic steering column and steering shaft are used as the steering column 2 and steering shaft 3 so that it is possible to adjust the forward-backward position of the steering wheel 4.
In recent years, various kinds of anti-theft apparatuses have been installed in automobiles as a countermeasure against theft of the automobile. One kind of such an anti-theft apparatus is a steering lock apparatus that makes it impossible to operate the steering wheel unless a proper key is used. FIG. 9 illustrates an example of a steering lock apparatus that is disclosed in JP 2008-265646 (A). In a steering lock apparatus 10, with a guide section 13 that is provided on the tip end section of a lock unit 12 arranged inside a lock hole 11 that is formed in the intermediate section in the axial direction of the steering column 2a, the lock unit 12 is supported by and fastened to the steering column 2a. Moreover, a key-lock collar 15 having an engaging concave section 14 formed in at least one location in the circumferential direction thereof is fastened around a location of part of the steering shaft 3a that is in phase in the axial direction with the lock unit 12. During operation (during key lock), the tip end section of a lock pin 16 of the lock unit 12 displaces toward the inner-diameter side of the steering column 2a and engages with the engaging concave section 14, such that the rotation of the steering shaft 3a is essentially prevented with force about the size of that for operating the steering wheel 4 in the normal driving position. However, when the steering wheel 4 is rotated by force that exceeds a value that is regulated by the key lock regulation even in a case where the engaging concave section 14 is engaged with the lock pin 16, the steering shaft 3a rotates with respect to the key-lock collar 15. In a steering lock apparatus with this kind of structure, the key-lock collar 15 and steering shaft 3a are not damaged even though the steering wheel is rotated by large force with the ignition key in the OFF state.
On the other hand, being influenced by energy conservation in recent years, and in order to improve fuel consumption performance and reduce costs, there is a need to reduce the weight of the steering column 2a by making the thickness of the steering column 2a thinner, or the like. When the thickness of the steering column 2a is made thinner, there is a possibility that the anti-theft function will be lost when using the structure disclosed in JP 2008-265646 (A). In other words, when attempting to rotate the steering wheel 4 by large force with the ignition key turned OFF and the engaging concave section 14 of the key-lock collar 15 engaged with the lock pin 16, before the steering shaft 3a can rotate with respect to the key-lock collar 15, a large force is applied from the lock pin 16 to the circumferential edge section of the lock hole 11 by way of the guide section 13 due to a large friction force that is greater than the dynamic friction force. Due to the large force that is applied to the circumferential edge section of the lock hole 11, there is a possibility that damage such as cracking of the steering column 2a, which is made of a light alloy material such as an aluminum alloy, or carbon steel for machine structure having a thin wall thickness, will occur. In that case, there is a possibility that the lock hole 11 and the guide section 13 will become disengaged, and that the lock unit 12 will rotate around the steering column 2a. In this state, engagement of the engaging concave section 14 and the lock pin 16 easily becomes disengaged, however, when this engagement actually becomes disengaged, the steering shaft 3a rotates on the inner-diameter side of the steering column 2a, so the anti-theft function is lost.
In order to solve this kind of problem, JPH 08-295202 (A) discloses a structure in which a convex section of the lock unit that is formed on a butting surface that comes in contact with the outer circumferential surface of the steering column fits in a through hole that is formed in the steering column such that there is no movement. With this kind of structure, the force that is applied to the steering column from the steering wheel is distributed, and it is possible to suppress the occurrence of damage to the steering column. However, with this structure, the through hole that passes through in the radial direction through the steering column is provided on the circumferential edge section of the lock hole of the steering column, so the strength and rigidity of the circumferential edge section of the lock hole are decreased to some extent, and there is a possibility that a sufficient effect of preventing damage will not always be obtained.